chore: simplify proof tests

[jackchuma]

Simplifies and consolidates the L1 proof test suite.

Changes

• Reduces test file sizes significantly (~1000 lines removed)
• Standardizes visibility modifiers (public → internal) for test fixtures
• Removes unused imports and dead code
• Bumps pragma to ^0.8.15 for consistency
• Tightens up test setup and helpers across all proof test files

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1